Large quantities of di- and polyisocyanates are produced each year. The largest part of isocyanate production is destined for the preparation of urethane group-containing polymers, especially cellular and non-cellular polyurethanes. Several isocyanates are produced commercially but by far the largest quantities are mixtures of 2,4- and 2,6-toluene diisocyanates, and the various isomers of diphenylmethane diisocyanate and their mixtures, plus its higher polymeric homologs (polymeric MDI). Since the first isocyanate synthesis by Wurtz in 1854 in which methyl isocyanate was produced from dimethylsulfate and potassium cyanate, many methods have been developed for the production of isocyanates. However, the only commercially feasible method has been the phosgenation of amines or amine salts, a reaction practiced by Hentschel at least as early as 1884.
The industrial process for the production of isocyanates suffers from several major disadvantages. Central to these disadvantages is the use of phosgene. Phosgene is exceptionally toxic and corrosive making its handling hazardous. Furthermore, the manufacture of phosgene, often performed on site, is also hazardous, as it involves the reaction of carbon monoxide with chlorine. The necessity of having a captive supply of chlorine available makes phosgene production expensive, due to the high cost of electricity used in the preparation of chlorine. The cost of chlorine production is also subject to severe market fluctuation because of the necessity to sell or otherwise dispose of large amounts of caustic or its equivalents. In addition to these disadvantages relative to the cost, danger and difficulty of handling, the use of phosgene suffers from another disadvantage in that two moles of by-product hydrogen chloride are produced per mole of isocyanate group formed. This large amount of hydrogen chloride, generally in the form of the acid, must be recycled, sold or disposed of as waste.
Because of the foregoing disadvantages of the phosgene process for isocyanate production, there has been a long-felt need for a phosgene-free method. This need was expressed more than 25 years ago in U.S. application No. 757,907 of 1958, which matured into U.S. Pat. No. 3,054,819 on Sept. 18, 1962. The patentees indicated that several non-phosgene processes had been developed but that none had met with commercial success. In particular, the thermal cleavage of N-substituted carbamic acid esters was discussed. While phosgene was not necessary to prepare isocyanates from N-substituted carbamates, unfortunately, phosgene was necessary to prepare the carbamates themselves. Thus, the use of phosgene was not eliminated but merely shifted to an earlier process step. The process of U.S. Pat. No. 3,054,819 involved the pyrolysis of N-substituted carbamates formed by reaction of an amine with diethylcarbonate. However, as with the industrial preparation of N-substituted carbamates, the manufacture of diethylcarbonate also requires the use of phosgene. Thus again the use of phosgene is not eliminated but merely shifted to another process step.
In U.S. Pat. No. 3,467,687 an aromatic nitroso compound is reacted with carbon monoxide in the presence of a suitable catalyst to produce isocyanates. However, this process requires high pressure and the use of both noble metal and Lewis acid catalysts. Furthermore, while eliminating the use of phosgene and its attendant disadvantages, the process does involve the use of carbon monoxide, itself a highly poisonous and corrosive gas. Because of these difficulties, this process has not been commercialized.
Several processes have been developed utilizing the reaction of carbon monoxide with an aromatic nitro compound. Illustrative is U.S. Pat. No. 3,481,967, patented Dec. 2, 1967. While this process suffers from the same disadvantage of the use of carbon monoxide as previously discussed, an advantage is the avoidance of the necessity to reduce the aromatic nitro compound to the corresponding amine. Hence, this process would appear to have some attractive economic advantages. Unfortunately, the separation of toxic catalysts (generally selenium in the modern versions of this process) from the product has proven difficult. Hence, this process also has not been utilized commercially.
A commercially feasible, non-phosgene process for the production of isocyanates, therefore, has thus far eluded the chemical industry. lt is an object of this invention to enable the non-phosgene production of isocyanates through the preparation of N-aryl substituted carbamate esters (hereinafter N-aryl urethanes) in high yield, by a process which does not involve phosgene or carbon monoxide in any step. Nor does the process involve the use of highly toxic metals as catalysts. These N-aryl urethanes may then be thermally cleaved to isocyanates by known processes as, for example, described in U.S. Pat. No. 3,919,278.